Install Flatpak & Friends

Flatpak is a cross-distribution framework for distribution and running of
sandboxed applications.
XDG Desktop Portal provides Nuvola Apps with desktop integration.
GNOME Software allows you to browse and install flatpaks in a more user friendly way.
It is needed because elementary's AppCenter still doesn't support flatpaks.

All packages are available in the elementaryOS 5 Juno repository. No third-party package archive is required.

Launch AppCenter and search for "flatpak".

Install the package "Flatpak Support" ("Flatpak is a framework for desktop applications on Linux").

Restart your computer

It is necessary to restart your computer to add Flatpak paths alongside the system ones and to activate
the Ubuntu Software Flatpak Plugin. Otherwise, you may see strange errors.
Unfortunately, just logging out and logging in is not enough.

Add Flathub and Nuvola Apps Repositories

The Flathub repository provides GNOME Platform runtime, which is used by Nuvola to share common
dependencies with other Flatpak apps to reduce disk & memory usage. The Nuvola Apps repository
contains Nuvola Apps Service and individual Nuvola web apps.

Install Nuvola Apps Service

Nuvola Apps Service is an optional background service that provides individual Nuvola apps with
global shared resources such as a global configuration storage, global keyboard shortcuts, a HTTP remote control
server, and a command-line controller. While this service is optional, the installation is recommended.

In order to install Nuvola Apps Service with GNOME Software, launch GNOME Software,
search for Nuvola Apps Service and finally click the Install button.